1. Field of the Invention
The present invention relates to a radio receiver for data communication of the kind handling digital signals.
2. Description of the Prior Art
A radio receiver of the type using a double superheterodyne system is one of conventional receivers applicable to data communication which handle digital signals. This type of receiver, or double superheterodyne receiver, has a radio frequency section for selecting an electromagnetic wave which comes in through an antenna. A first intermediate frequency converting section removes a carrier from a radio frequency (RF) signal selected by the radio frequency section and thereby converts it to a first intermediate frequency (IF) signal. A first bandpass filter filters out an image frequency and other undesired frequency components contained in the first IF signal and resulting from frequency conversion. The first bandpass filter has customarily been implemented by a crystal monolithic filter having an excellent narrow band characteristic.
The IF signal coming out of the first bandpass filter is further converted by a second intermediate frequency converting section to a second IF signal containing a desired signal waveform. A second bandpass filter picks up the desired signal waveform out of the second IF signal. It is a common practice to implement the second bandpass filter as a four-element ladder type ceramic filter. An AGC (Automatic Gain Control) amplifier automatically adjusts the gain of the signal waveform having been produced by the second bandpass filter. The amplified signal waveform is applied to another filter which is also implemented as a four-element ladder type ceramic filter, whereby spurious frequencies and higher harmonics which the previous ladder type filter failed to attenuate are removed. The resulting signal waveform, free from spurious frequencies and higher harmonics, is again amplified by an amplifier. This amplified signal waveform is detected by a detector to become a data signal. An equalizer corrects the delay of the data signal ascribable to the two successive ladder type filters and then transfers it to a data processing section.
The conventional double superheterodyne receiver having the above construction has various problems left unsolved, as follows. The first bandpass filter constituted by a crystal monolithic filter as stated causes spurious emissions to occur due to mechanical oscillations thereof. Since such a first bandpass filter is not capable of attenuating spurious frequencies to a satisfactory degree, both the second bandpass filter and the filter which follows it have to be implemented by a four-element ladder type ceramic filter in order to fully remove the spurious components. The drawback with a ladder type filter is that it is complicated in construction and expensive. Moreover, a four-element ladder type filter inherently delays a signal passing therethrough. The use of two four-element ladder type filters, therefore, aggravates the delay, resulting in the need for a complicated equalizer.